1. Field of the Invention
The present invention relates to a GaN substrate, a substrate with an epitaxial layer, a semiconductor device, and a method of manufacturing the GaN substrate, and more particularly, to a GaN substrate, a substrate with a epitaxial layer, a semiconductor device and a method of manufacturing the GaN substrate capable of making use of a semipolar plane.
2. Description of the Related Art
Conventionally, a GaN-based laser diode (LD) and a GaN-based light emitting diode (LED) have been known. The GaN-based LD and LED have been formed by depositing an epitaxial layer on a (0001) plane of a sapphire substrate, a SiC substrate, or a GaN substrate. Since the (0001) plane of the above GaN substrate, for example, is a polar plane, there has been a problem that the luminance efficiency of the LED is lowered in a long-wavelength region having an emission wavelength of 500 nm or more.
To solve the above problem, it is reported that the luminance efficiency in the above long-wavelength region is improved by forming a quantum well structure on a semipolar crystal plane such as a (11-22) plane instead of the conventional (0001) plane in a GaN crystal (refer to “News Release: Succeeded in LED development on semipolar-plane bulk GaN substrate”), [online], Jun. 30, 2006, Kyoto University [retrieved Jun. 1, 2007], Internet (http://www.kyoto-u.ac.jp/notice/05_news/documents/060630—1.htm) (Non-patent Document 1)). In addition, a method of manufacturing a GaN substrate in which such a semipolar crystal plane is exposed to a main surface has been proposed (refer to Japanese Patent Laying-Open No. 2005-298319 (Patent Document 1).
In addition, in order to improve the operating life and performance of a light emitting element such as a GaN-based LED, it is proposed to form a defect concentrated region in which a defect such as dislocation is collected in a GaN crystal to reduce a defect density of a region around the defect concentrated region (refer to Japanese Patent Laying-Open No. 2003-183100 (Patent Document 2)).
According to an LED disclosed in the above Non-patent Document 1, a semipolar crystal plane formed naturally as a microfacet is used, and the crystal plane is fixed to the (11-20) plane and small in size. However, in view of efficient manufacturing of LEDs and LDs, it is preferable to manufacture an LED and the like with a GaN substrate having a large diameter of 2 inches or more in which a semipolar crystal plane is exposed to a main surface (that is, a predetermined plane direction ([0001] direction, for example) is inclined in a predetermined direction at a predetermined angle, that is, has an off-angle with respect to a normal vector of the main surface). In addition, when the value of the inclined angle of the plane direction with respect to the normal vector of the main surface is adjusted (that is, when the crystal plane exposed to the main surface of the substrate is changed), it is considered that the characteristics of the LED and LD can be improved. Thus, when the GaN substrate in which the above-described defect concentrated region is formed and the defect density of another region is reduced is used, and the value of the inclined angle of the plane direction with respect to the normal vector of the main surface is adjusted as described above, it is considered that the characteristics of the LED and LD can be improved.